Lunar launch system

This is the brainstorm that I was thinking about.  The idea was to use Fresnel lenses in order to heat up a reaction mass for thrust.  The reaction mass would be hydrogen obtained from lunar water, presumed to be found in permanently shadowed craters near the polar regions.

After considering this for a little while, cold reality began to make its presence known.  The question arose: what is the maximum temperature that can be reached by such a device?  I'm not sure.  It seems apparent from the video link above that temperatures of over 1000 Farenheit are possible.  But is this enough and is this the limit?

That's a question I can't get an answer to very quickly.  So, I am dropping that idea in favor of another proposition, which would include heating the reaction mass.  There doesn't appear to be much doubt that you could get sufficient power to a device that could heat up some hydrogen to levels of a nuclear thermal rocket.  If you could do that, you could get an Isp of around 800 or 900, which is nearly twice that of the recently retired Space Shuttles' main engines.  Not bad for a compact device, and it would take no nukes to do it.

I recalled Parkins' device, which I covered previously here.  

In the comments section, there's a mention of the NERVA nuclear thermal rocket, which was scheduled to be tested on Apollo 20, which was canceled.  It was space ready.

Back to Parkins: It appears that his device used what is called gyrotrons in order to produce the microwave energy which heats up a heat exchanger.  Hydrogen is passed through it and expelled through a nozzle for thrust.

What if a gyrotron could be made small enough so that it could heat up hydrogen in this manner so that it could be used for thrust in a spacecraft?  I looked up gyrotrons on the wikipedia.  On portable application for these devices is in the Active Denial System for the military.

http://en.wikipedia.org/wiki/File:Active_Denial_System_Humvee.jpg

Could such a device could be modified to make it useful for rocket propulsion?  I think the answer could be yes.  Another question is its mass and how much energy does it take and how much does it make?

Mass is important because it imposes great penalties on a rocket.  Another question is the loss in that penalty worth it in terms of performance?  If a rocket could get a 900 Isp with a thing like this, you may be willing to pay for that performance especially since you won't have the mass penalty of having to carry oxygen on board.

Parkins device kept the gyrotrons on the ground, but that imposes its own limits.  If you had a portable one onboard, you could generate your own thrust from solar power and by using hydrogen as a reaction mass. The advantage of hydrogen is that it can enable rocket engines of high Isp to be constructed.   With the gyrotrons, you eliminate the nuclear power component, which has the advantage in that that you won't need radiation shielding.

Could a device such a this have enough thrust to get off the moon?  I'm still thinking about the subject.  Back to that later.

Update:

I'm not sure about the Apollo 20 reference.  But the Nerva nuclear thermal rocket was deemed space worthy.  It could have flown.   It was canceled because it was feared that it would be a commitment to an expensive foray to Mars.  Therefore, it was not due to a failure, but to its success.  The government was afraid of it, in other words.